The Equatorward Boundary of the Auroral Current System During Magnetic Storms.

Our current knowledge of the geomagnetic poleward and equatorward boundary dynamics is limited, particularly, how deep those two latitudinal boundaries can extend into lower geomagnetic latitudes during magnetic storms. We want to understand the motion of the boundary because it is important in terms of the location and magnitude of the effects of geomagnetic disturbances associated with storms on the ground. In this study we derive spherical elementary ionospheric currents from ground magnetometer arrays covering North America and Greenland during six magnetic storms in 2015 and 2018. With two dimensional maps of the auroral region current, we select the equatorward boundary of the region 2 currents by‐eye and fit the boundary with an ellipse to derive the location of the equatorward boundary at magnetic midnight. We have obtained over 500 boundaries and find that the midnight boundary location varies between 45° and 66° magnetic latitude. We examine the influence of the interplanetary magnetic field (IMF), solar wind plasma, and geomagnetic indices on the location of the magnetic midnight equatorial boundary and find that the equatorial boundary location is best correlated with the IMF Bz, VBz, and the Sym‐H index. We demonstrate that as the Bz component becomes more negative, the magnitude of VBz increases, and the magnitude of the Sym‐H index increases, the magnetic midnight equatorial boundary shifts equatorward during periods of moderate to high geomagnetic activity. Plain Language Summary: Our current knowledge of the geomagnetic poleward and equatorward boundary dynamics is limited, particularly, how deep the equatorward boundary of the auroral current system can extend into lower magnetic latitudes during magnetic storms. In this study we use two dimension ionospheric current maps derived from ground magnetometer arrays covering North America and Greenland during magnetic storms in 2015 and 2018 to understand the variability of the equatorward boundary of the auroral currents at magnetic midnight. We examine the influence of the interplanetary magnetic field (IMF), solar wind plasma, and geomagnetic indices on the location of the magnetic midnight equatorial boundary and find that the magnetic midnight equatorial boundary location is best correlated with the Bz component of the interplanetary magnetic field, the solar wind electric field (VBz), and the Sym‐H geomagnetic index. We demonstrate that as the Bz becomes more negative, the magnitude of VBz increases, and as the magnitude of the Sym‐H increases the magnetic midnight equatorial boundary moves equatorward. Key Points: We show the temporal and spatial development of the region 2 current system equatorward boundary during magnetic stormsWe show that the equatorward boundary at magnetic midnight varies in magnetic latitude between 45° and 71° MLatWe demonstrate a correlation between the equatorward boundary location and IMF Bz southward, VBz, Sym‐H, and the ε parameter [ABSTRACT FROM AUTHOR]